1. Field of the Invention
This invention relates to a gas sensor. More particularly, the invention relates to a gas sensor for detection of reducing gases contained in air, exhaust gas, expired air or other test gases, such as hydrocarbons and their derivatives, e.g., propane, methane, butane, ethylene, alcohols, carbon monoxide, hydrogen, carboxylic acids and amines.
2. Description of the Prior Art
Detection of reducing gases contained in air, exhaust gas and expired air, such as mentioned above, has heretofore been conducted according to various methods, for example, gas chromatography, chemical analysis and nondispersive infrared absorption analysis. These methods, however, are defective in various points. For example, a large and expensive apparatus should be used, or experiences and skills are required for measurement or maintenance of measurement equipment. As means capable of overcoming these defects, there has been proposed a gas sensor comprising a material, the electric resistance of which changes greatly when it falls in contact with a reducing gas such as mentioned above. A so-called reduction-type oxide semiconductor such as SnO.sub.2, ZnO and Fe.sub.2 O.sub.3 is generally used as the gas sensible material in the gas sensor of this type. In many cases, in order to improve the response characteristics or the sensitivity in these gas sensors, a catalyzer is incorporated, and further, a recovering agent is added to restore the electric resistance after the measurement to the original electric resistance before the measurement.
These conventional gas sensors comprising a reduction-type oxide semiconductor such as mentioned above are advantageous over the above-mentioned conventional detection techniques in that they are very cheap and the measurement can be accomplished very easily when they are used. However, incorporation of a catalyzer or a recovering agent is indispensable for the reasons set forth above. Further, the sensitivity or restorability of the gas sensor varies greatly according to the kind or amount of the catalyzer or recovering agent. Therefore, it is very difficult to obtain gas sensors having certain uniform characteristics, and application fields of the gas sensor of this type are limited.
In addition, the above-mentioned gas sensor comprising a reduction-type oxide semiconductor is defective in that the value of the electric resistance is greatly influenced by moisture in the atmosphere and an erroneous operation is often caused by the change of the humidity. Accordingly, an improvement is desired in this connection also.